Journal of Applied Bacteriology Symposium Supplement 1992,73, 9-1

02s

Antibiotics and the selection of food-borne pathogens E.J. Threlfall Division of Enteric Pathogens, Central Public Health Laboratory, London, UK

1. Introduction, 96s 2. Incidence of food-borne pathogens, 96s 3. Incidence of salmonella infections, 1981 a n d 1990, 96s 4. Antibiotic resistance in salmonellas, 97s 4.1 Incidence of resistant strains isolated from humans: comparison of 1981 and 1990,97S 4.I . 1 Overall incidence, 97s 4.1.2 Resistance to individual drugs, 97s 4.1.3 Incidence of multiple resistance, 98s

4.2 Incidence of resistance in strains isolated from animals, 98s 4.2.1 Cattle, 98s 4.2.2 Poultry, 99s 4.3 Recent developments, 99s 4.3.1 Multiple drug resistance in Salmonella typhimurium from poultry, 99s 5. Antibiotic resistance in Campylobacter spp., 100s 6. Conclusions, 101s 7. References, 101s

1 . INTRODUCTION

described. T h e findings are discussed in relation to selection pressures exerted by the use of antibiotics in the foodanimal reservoirs of these pathogens.

The role of antibiotics in promoting the emergence and spread of drug-resistant pathogenic bacteria which infect humans through the food-chain has been a contentious subject for almost 30 years. In the late 1960s the use of antibiotics in animals to combat infections and promote growth was to a large extent being negated by the emergence of multi-resistant strains of Salmonella typhimurium in cattle. Increasing concern about the use of antibiotics in animals bred for food resulted in 1968 in the convening of the Joint Committee on the Use of Antibiotics in Animal Husbandry and Veterinary Medicine (the Swann Committee). Their report (Anon. 1969) has had considerable impact on the use of antibiotics, particularly for purposes of growth promotion. Whether the Swann recommendations had any real effect in controlling the emergence and spread of multi-resistant organisms in food animals is debatable. In the later 1970s multi-resistant salmonella strains of bovine origin caused many infections in humans (Threlfall et al. 1978a, b). This resulted in considerable controversy, not only about whether the Swann Committee had failed in its objectives (Anon. 1980) but even about its original concept, that antibiotic usage in food animals had indeed selected for drugresistant strains of potential importance to human health (Walton 1978). In an attempt to answer these questions, changes in the incidence of antibiotic resistance and in patterns of resistance in the most common food-borne pathogens causing infections in humans in England and Wales in the 1980s are Corrrspondence to : Dr E . J . Threlfirll, Dicisron of Enterrc Pathofens, Central Public Ilt~ultlrLrrboratnr),, hl Colindalr .4renite, London .VWY S H T , C,'K.

2. INCIDENCE OF FOOD-BORNE PATHOGENS

In England and Wales salmonellas and campylobacters are the most common food-borne pathogens. In 1990, 30 112 salmonellas isolated from humans were identified by the Division of Enteric Pathogens (DEP) of the Public Health Laboratory Service (PHLS) (Threlfall et al. 1991a). Likewise in 1990 34555 laboratory reports of human campylobacter isolations were made to the Communicable Disease Surveillance Centre of the PHLS (CDSC, personal communication-provisional figures only). Long-term epidemiological studies have shown that the food-animal hosts of the most common salmonella serotypes in humans in England and Wales are poultry and cattle and, to a lesser extent, pigs (Palmer & Rowe 1986). For campylobacters both poultry (Doyle 1984; Rosef et al. 1984; Anon. 1990) and cattle (Bolton et al. 1985) have been implicated as important animal reservoirs. Substantial outbreaks associated with contaminated raw or improperly pasteurized milk have also been described (Jones et al. 1981). 3. INCIDENCE OF SALMONELLA

INFECTIONS, 1981 AND 1990

In 1990 the most common serotypes in non-typhoidal salmonellas from humans were Salm. enteritidis, Salm. typhimurium and Salm. &chow. These comprised 63'10, 18% and 1"/0 of total isolations (Threlfall et al. 1991a). The

ANTIBIOTICS A N D FOOD-BORNE PATHOGEN SELECTION 97s

remaining 15% were made up of 193 different serotypes. Some of these accounted for ca 1% of the total isolations (e.g. Salm. heidelberg, Salm. panama, Salm. kedougou, Salm. berta). The majority, however, comprised only a few strains, each serotype comprising less than 0.1% of total number of strains received. In 1981 the corresponding figures were 8829 strains; 12.3% Salm. enteritidis, 45.2% Salm. typhimurium, 7.5'y0 Salm. virchow and 35% other serotypes (188 serotypes) (Ward el al. 1990). In addition to identifying human salmonellas, the DEP also phage types Salm. typhimurium, Salm. enteriridis and Salm. oirchow isolated from food-animals and referred to them by laboratories of the Veterinary Investigation Service. 4. ANTIBIOTIC RESISTANCE I N SALMONELLAS

Salmonellas from both humans and food-animals are screened for resistance to a range of antimicrobial agents : ampicillin (A), chloramphenicol (C), gentamicin (G), kanamycin (K), streptomycin (S), sulphonamides (Su), tetracyclines (T), trimethoprim (Tm), furazolidone (Fu) and nalidixic acid. This permits trends in the Occurrence of resistance to different antimicrobial agents to be documented. It also provides a background for the investigation of factors that have contributed to changes in the incidence of resistance. 4.1 Incidence of reslstant strains Isolated from humans: comparison of 1981 and 1990

4.1.1 Overall incidence

The overall incidence of drug resistance (i.e. resistance to one or more antimicrobial agent) of Salm. enteritidis has decreased slightly in 1990, from 15% to 11%. It has

Table 2 Drug resistance in salmonellas in humans, England and Wales 1981 and 1990. Percentage resistance to individual

drugs

Table 1 Drug resistance in salmonellas in humans, England and Wales 1981 and 1990. Occurrence of resistance

Serotype

Salmonella enteritidis

YoDR %MR %DR %iMR %DR YOMR %DR Y O MR

typhimurium virchow Others

1.5

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Antibiotics and the selection of food-borne pathogens.

Journal of Applied Bacteriology Symposium Supplement 1992,73, 9-1 02s Antibiotics and the selection of food-borne pathogens E.J. Threlfall Division...
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